This invention relates to an ophthalmoscopic photographing and observing system with an eye fundus illuminating optical system and more particularly to a system which eliminates scattering rays of light produced inside and at the rear face of the crystalline lens of an eye which is being examined.
Illumination rays of light that are reflected by a corneal surface cause a flare in a photographic image when they mix with a photographing light. To prevent such inconvenience, a ring-shaped aperture stop has been previously provided within an illuminating optical system of an eye fundus (or retinal) camera at a position conjugate to the corneal surface, as described in a British Pat. No. 799,812.
In accordance with such a method of removing the reflected light, a ring-shaped illuminating light beam is projected upon the eye through the peripheral area of the pupil of the eye (that is, an aperture of the iris). The photographic beam of light reflected by the fundus of the eye is allowed to pass through the middle part of the pupil. An area on the cornea through which one of the two light passes is thus arranged to overlap another area of the cornea through which the other light flux passes, so that the undesired light reflected by the cornea is removed.
A U.S. Pat. No. 3,851,954 discloses another type of arrangement, wherein the reflection of the eye fundus illuminating light is prevented at the front and also at the rear of the crystalline lens of an eye being examined. A center obscuring stop is positioned either conjugate to the front face of the crystalline lens or conjugate to an intermediate point between the front face and the rear face of the crystalline lens.
The present inventors have discovered an ophthalmoscopic camera that permits photographing without the use of mydriatics.
The ophthalmoscopic camera which does not require the use of mydriatics differs from the conventional ophthalmoscopic camera in that the diameter of the above stated ring-shaped aperture is smaller for the former than for the latter. The reason for this is that the diameter of a pupil is smaller when no mydriatics are used than when a mydriatics is used. When they attempted to use the above-mentioned new camera, the present inventors found a flare remaining in the image obtained. It was also discovered that an illuminating light beam directed toward the fundus of an eye was scattered by the film of crystalline lens of the eye and by the inner liquid thereof such that scattering of the light caused the flare in the image. Since the liquid inside a crystalline lens usually becomes muddier with advance of age, such scattered light increases in aged people.
It is also conceivable that such a flare in an image is caused by the increased intensity of a flash light due to a smaller diameter of the ring-shaped aperture and by a crystalline lens passing optical path which is closer to the optical axis due to the smaller diameter of the ring-shaped aperture.
In the accompanying drawings, FIG. 5 illustrates an eye being examined. Here, reference symbol P indicates a cornea; Q indicates a crystalline lens; Q1 indicates the front face of the crystalline lens and Q2 indicates the rear face thereof. A reference symbol C indicates an image of a ring-shaped aperture, a part indicated by hatching representing its shaded area. An illuminating light passes through the areas between the pairs of lines L and l, while a photographing light passes between lines l. In order to prevent the illuminating light from being reflected at the part R on the front face Q1 of the crystalline lens, an image D of a center obscuring stop is formed on the front face Q1. The light reflected by the rear face Q2 is thus removed by the shadow of the image D.
To prevent the illuminating light from being scattered through the crystalline lens, the image D of the obscuring stop must cover at least a scattering face S on the rear face Q2. For this purpose, if the image of the obscuring stop is made larger, the quantity of illuminating light would decrease substantially. To avoid such decrease in illuminating light, the size of the ring-shaped aperture must be increased accordingly and such arrangement causes inconvenience.